Will Dunham
WASHINGTON (Reuters) – Scientists have discovered the oldest known RNA, a molecule essential for most biological functions, in a woolly mammoth that inhabited Siberia about 39,000 years ago, proving it may have existed longer than previously thought and promising a new way to study organisms that lived long ago.
The RNA, successfully isolated and sequenced, was extracted from the muscle tissue of the left front leg of a young male mammoth, aged five to ten years, whose carcass was discovered in 2010 in Siberian permafrost in the Russian Far East along the coast of Oyogos Yar, bordering the Laptev Sea.
The mammoth, named Yuka, represents one of the best preserved frozen carcasses of this extinct species. His RNA revealed, among other things, which genes were “turned on” in Yuki's tissues at the time of death, showing signs of cellular stress.
Most knowledge about prehistoric organisms comes from the study of skeletal fossils, but there is a limit to what they can reveal about their biology. However, the growing ability to recover characteristic biomolecules from ancient remains has opened up new ways of understanding such organisms.
The new RNA advance complements recent advances in the study of ancient DNA, the molecule that carries the genetic instructions for all living organisms, and ancient proteins, the molecules that build and control much of a cell's machinery.
RNA, short for ribonucleic acid, serves as messages sent from an organism's genome to its cellular machinery, telling cells which genes to activate or deactivate, how and when to regulate their function, and which proteins to make.
“With RNA, you can access the actual biology of a cell or tissue, happening in real time in the last moments of an organism's life,” said Emilio Marmol, a geneticist, veterinarian and bioinformatician at the University of Copenhagen's Globe Institute and lead author of the study published Friday in the journal Cell.
“This gives us direct access to the functional landscape of the cellular metabolism of woolly mammoths when they were alive, which is not possible – at least at the scale we report – simply using DNA or proteins. Adding this layer of information provides a more complete understanding of the biology of woolly mammoths,” Marmol said.
RNA is more fragile than DNA and proteins. The oldest DNA discovered to date comes from animals, plants and microbes and dates back to about 2 million years ago from sediments in Greenland. The oldest squirrels come from the teeth of a hornless rhinoceros that lived about 23 million years ago in the Canadian Arctic.
Until now, the oldest RNA discovered belonged to a wolf cub that lived in Siberia about 14,000 years ago.
“I think the key discovery is that we can recover RNA from such an old sample. This is a proof of principle that opens the door to much more detailed analysis of gene expression patterns in Ice Age megafauna,” said geneticist and study co-author Lav Dahlen of Stockholm University and the Center for Paleogenetics in Sweden.
Before Yuka died, he was injured: deep cuts in the skin indicated an attack by a cave lion, an extinct species that was a cold-adapted and larger relative of the modern African lion.
“None of the marks are serious enough to cause death, so it is still unclear why Yuka died,” Dahlen said.
Researchers found RNA molecules in Yuki that code for proteins involved in muscle contraction and metabolic regulation under stress, possibly, but not necessarily, caused by the attack.
Scientists have long believed that RNA degrades just minutes or hours after death, but this and other studies have shown that under the right circumstances it can survive much longer. The cold conditions of Siberia were particularly conducive to the preservation of RNA.
Yuki's DNA had also been sequenced by these and other researchers previously. Previously thought to be female, genetic evidence showed that Yuka, who stood about 5-1/4 feet (1.6 meters) at the shoulder, was male with an X and a Y chromosome.
The researchers believe that under the right conditions, RNA can be recovered from even more ancient remains, noting that ancient RNA opens up many possibilities.
“We hope our work will spark new interest in studying RNA in other old remains, not necessarily from the Ice Age or extinct species, but also in medieval or historical remains of both extinct and extant organisms,” Marmol said.
(Reporting by Will Dunham; Editing by Daniel Wallis)
